This invention relates generally to laser systems, and, more specifically, to optical alignment of elements in a laser resonator.
A laser system includes a laser resonator or cavity containing a suitable laser material which may be in the form of a crystal or glass optically aligned between a pair of opposing optical elements such as lenses or mirrors. The laser material may be optically pumped by arc lamps or other lasers, for example, for generating a laser beam in the resonator. The resonator may be operated in continuous wave (cw) or pulsed modes of operation. The laser resonator design becomes more complex as the time duration of the laser pulses become smaller, with femtosecond (fs) pulsed lasers typically being the most sophisticated and complex.
Typical laser resonators require various other optical elements for obtaining suitable performance in specific applications of increasing complexity. Each optical element of the laser system typically has five degrees of freedom which must be accurately controlled to achieve proper alignment. Each element may be mounted in a fixture that allows positioning of the elements along three orthogonal coordinate axes, with two degrees of tilt. As more optical elements are required in the laser system, the complexity of achieving accurate optical alignment of all of the elements increases. For example, it may take several months to properly adjust all of the individual optical elements in a sophisticated pulsed laser resonator in order to achieve stable operation with suitable performance. Such adjustments are often made by trial and error, which leads to the lengthy setup time.
Accordingly, it is desirable to reduce the time required for optically aligning the various elements in a laser resonator to effect suitable resonator stability and desired performance.
Laser cavities frequently employ flat-surfaced elements tilted at Brewster's angle to allow for a lossless, intracavity propagation of one polarization mode. However, this requires use of powered elements (focusing or defocusing) in the cavity that are tilted to compensate for coma and astigmatism that are induced by the Brewster elements. Since the stability and spatial symmetry of the output laser mode are sensitive to the amount of aberration compensation, the tilt of powered elements must be accurately determined and set in order to maintain a symmetric (or TEM.sub.00) output mode that is insensitive to cavity misalignments.
Laser operators most commonly align each tilted cavity element by calculating the required orientation of the element (to compensate for induced aberrations by other elements) and placing the element in the cavity at the calculated angle. The element is then adjusted to maximize the output laser power or minimize the threshold for laser oscillation. However, this method only implicitly determines alignment of the element since an accurate tilt measurement is never made. This is because absolute angle measurements are usually impossible to make since the relative angular orientation of two cavity elements in some common coordinate system is rarely known. Consequently, a configuration that optimizes the laser power may not result in a symmetric (or TEM.sub.00) output mode. Even if the output power is maximized, laser performance can be adversely affected by small perturbations in the cavity configuration.
Sometimes, a trial and error method is employed by looking at different tilt adjustments of the optical element of interest. This inherently time-consuming method requires that the cavity be realigned for each tilt setting. Unfortunately, the results of this method are difficult to interpret because of difficulty in distinguishing a lack of aberration compensation from an alignment error.
Solid-state laser resonators typically operate with a transverse (spatial) beam mode which explicitly depends upon the optical properties of the laser crystal under the applied thermal and optical load of the pump and oscillating laser beams. The stability of a particular transverse beam mode determines whether or not that beam can be supported by the laser resonator and generate useful output power. Usually, more than one transverse beam mode is stable, and the resonator output energy comprises a linear superposition of several modes, weighted by the net gain experienced by each mode. If one mode is strongly gain favored, it may undesirably drive the gain for the other modes below the laser threshold, resulting in output energy of a single transverse mode.
When the optical properties, including focal power, of the laser crystal are modulated by the instantaneous intensity of the applied laser fields through a nonlinear refractive index, a specific temporal format for the output transverse laser mode may be favored. Since the desired applications of the laser output mode usually constrain the temporal format and frequency content of the laser radiation to particular ranges of pulse duration and linewidth, it is desirable to modify the design of the laser resonator and constrain adjustable values such as instantaneous pump power, oscillating bandwidth, output coupling, etc., so that the desired transverse mode and temporal format is significantly favored over competing transverse modes and temporal formats.
It is also desired to address resonator designs in which the coupling of the transverse, intracavity mode symmetry to the laser-induced focal power in the laser crystal discriminates between the true cw (continuous wave) and continuously pulsed operating formats, especially the true cw and passive mode-locked operating format of cw-pumped solid-state lasers, and between true cw, passive mode-locked, and forced mode-locked operating formats in synchronously-pumped solid state lasers. Passive and forced modelocking in synchronously pumped lasers are distinguished by the absence or presence of a constant temporal phase relationship between the pulses of the pump and oscillating lasers.